I have got the 710 over here so it's the telecom version, but I am running with a 2degrees sim at the moment, so not sure if that would be causing the problem, I am in Christchurch so pretty sure I am running off the 2degrees towers, and get internet fine.
I also got impatient while waiting for the tango update and manually flashed it, which could have installed breaky things but still got the message from Zune there was an update available a couple of weeks ago, so did that. Never tried tethering it until yesterday and just get the error when ever I try.
Just wondering if there is anyone else running the telecom 710 on 2degrees that has been able to get tethering working? If so I might try rolling back to preupdating, otherwise I guess I have a xt sim here, and the new $20 a month prepaid deal doesn't look too bad :)